This invention claims priority from Japanese Patent Application Number JP2006-229280 filed on Aug. 25, 2006, the content of which is incorporated herein its entirety.
1. Field of the Invention
The present invention relates to a junction field effect transistor (junction FET) and a method of manufacturing the junction FET, and particularly to a junction FET in which a high frequency characteristic and a noise figure characteristic thereof are improved, and a method of manufacturing the junction FET.
2. Description of the Related Art
In a conventional junction FET, for example, an n type well region, which serves as a channel region, is formed on a p type semiconductor substrate. In the n type well region, an n+ type source and an n+ type drain regions are formed. Then, a gate region is formed between the source and drain regions. This technology is described, for instance, in Japanese Patent Application Publication No. Hei 08-227900 (p. 2, and FIG. 6).
Description will be given of a conventional junction FET 200 with reference to FIG. 8A and FIG. 8B. FIG. 8A is a plan view showing the conventional junction FET 200. FIG. 8B is a cross-sectional view taken along the line b-b in FIG. 8A.
A p type epitaxial layer 22 is grown on a p type substrate 21, and then an n type epitaxial layer 24′ is formed thereon. A p+ type isolation region 23, which reaches the p type substrate 21, is formed to define and surround an n type well region 24, thereby the n type well region 24 comes to be a channel region 24.
An n+ type source region 25 and an n+ type drain region 26 are formed in the surface of the channel region 24. A source electrode 29 and a drain electrode 30 are respectively connected to the source region 25 and the drain region 26 through contact holes formed in an insulating film 40. In addition, a gate region 27 is formed between the source region 25 and the drain region 26.
Description will be given of a method of manufacturing the conventional junction FET 200 with reference to FIGS. 9A to 9D.
Firstly, the p type epitaxial layer 22 and the n type epitaxial layer 24′ are stacked on the p type substrate 21. The n type well region 24, which is to serve as the channel region, is isolated from the n type epitaxial layer 24′ by forming p+ type isolation region (ISO) 23 (FIG. 9A). An opening is formed in the oxide film 40 at a predetermined position, and a p type impurity is implanted and diffused in the n type well region 24 to form the p+ type gate region 27. The impurity concentration of the p+ type gate region 27 is an order of 1018 cm−3 (FIG. 9B). Thereafter, openings are formed in the oxide film 40 at predetermined positions where the source region 25 and the drain region 26 are to be formed. An n type impurity (for example, phosphorus (P)) is implanted and diffused in the n type well region to form the n+ type source region 25 and the n+ type drain region 26 (FIG. 9C). Furthermore, the source electrode 29 and the drain electrode 30 are formed to come into contact with the source region 25 and the drain region 26, respectively. Then, a gate electrode 31 is formed on the back side of the substrate (FIG. 9D).
It is important for the junction FET employed to an RF (high frequency) amplifier to have a high frequency characteristic. In the conventional junction FET 200, a depth d21 (see FIG. 8B) of the channel region 24 cannot be made shallow. For this reason, when the junction FET 200 is employed to an RF amplifier, the amplifier is generally used in a relatively low-frequency band, for example, 1 MHz.
Here, a cut-off frequency fT indicating the high frequency characteristic of the junction FET 200 is largely associated with the capacitance of pn junctions formed between the channel region 24 and the p+ type epitaxial layer 22 and between the channel region 24 and the p+ type isolation region 23. The reduction in the pn junction capacitance contributes to the improvement of the cut-off frequency fT.
Incidentally, as shown in FIG. 9, the conventional channel region 24 is isolated from the n type epitaxial layer 24′ by the isolation region 23. The n type epitaxial layer 24′ can be made as thin as approximately 2 μm in thickness. When the layer is made thinner than this limitation, it is hard to manage the variation in thickness of the epitaxial layer 24′ at the time of forming. Accordingly, there is a problem that characteristics of the channel region 24 are varied.
In other words, in the conventional structure, the area of the pn junctions formed between the channel region 24 and the p type epitaxial layer 22 and between the channel region 24 and the p+ type isolation region 23 is restricted by the thickness d21 of the n type epitaxial layer 24′ (depth of the channel region 24). As a result, the improvement in the high frequency characteristic of the junction FET 200 by reducing the pn junction capacitance cannot be achieved.
In addition, the improvement in the noise figure characteristic of the junction FET 200 has not been achieved either in the conventional structure. For the improvement in the noise figure characteristic, the reduction in the leakage current and the reduction in the internal resistance at an operation part are required. In the junction FET 200 of the conventional structure, however, generation of the leakage current in the pn junction portions formed between the channel region 24 serving as the operation part and the surrounding p type regions cannot be prevented.
Specifically, in the structure shown in FIG. 8, the channel region 24 is formed by causing the n type epitaxial layer 24′ to be isolated from the channel region 24 by having the isolation region (ISO) 23 therebetween. The gate region 27 comes into contact with the isolation region (ISO) 23 formed around the channel region 24, through which the gate region 27 is connected to the gate electrode 31 on the back side of the substrate. In order to reduce the input resistance of the device, the p type isolation region 23 serving as a current path has a high impurity concentration (1E19 cm−3 or higher). As a result, there is a large difference in the impurity concentrations between the channel region 24 and the isolation region 23, and the amount of the leakage current produced at the pn junction therebetween is accordingly increased.
Moreover, when the depth d21 of the channel region 24 is large as described above, the reduction in the internal resistance at the operation part is also interfered. An IDSS (or a pinch-off voltage) of the junction FET 200 is determined by the following factors: a depth d22 of the region immediately below the gate region 27 (depth of the portion from the bottom of the gate region 27 to the bottom of the channel region 24); the impurity concentration of the channel region 24; and a width (gate length) w21 of the gate region 27.
In other words, the depth d22 of the region immediately below the gate region 27 is automatically determined when a predetermined IDSS is ensured while keeping the gate length w21 and the impurity concentration of the channel region 24 constant. Thus, the determination of the depth d22 does not depend on the depth d21 of the channel region 24. Accordingly, when the depth d21 of the channel region 24 can not be set to be shallower than a certain depth (2 μm) as in the conventional structure, a depth d23 of the gate region 27 needs to be so large that a predetermined depth can be secured for the depth d22 of the region immediately below the gate region 27.
When the depth d23 of the gate region 27 is large, the length of the signal path which is formed between the source region 25 and the drain region 26 via a region immediately below the gate region 27 increases. Moreover, since the gate region 27 is formed by the diffusion of ions of the impurity, when the depth of the gate region 27 is large, the lateral diffusion therein (diffusion in a horizontal direction of the substrate) also progresses. Accordingly, the reduction in the signal path length cannot be achieved. For these reasons, the internal resistance is increased, and thereby, the noise figure characteristic is deteriorated.